Seat tilt mechanism - different types?

[M637and912]

I've been searching the net for a replacement tilt mechanism for my 1968 912.

MY tilt mechanism has 3 holes for the seat back and NO hooks underneath that hold the seat to the base (stop it tilting forward).

However, I've seen ones on the net that have two holes and/or a hook under the hinge to grab the lower base.

Do 912s come without the hook for the base or have my hinges been altered somehow?

I'm confused!